This invention relates to phased array ultrasonic transducers and, more particularly, to alignment of a dicing saw employed to define elements of an array ultrasonic transducer.
Array ultrasonic transducers, employed for example in medical applications, rely on wave interference for their beam forming effects, and typically employ a plurality of individual transducer elements organized as either a one-dimensional (linear) array or a two-dimensional array. Ultrasound is used as a non-invasive technique for obtaining image information about the structure of an object which is hidden from view, and has become widely known as a medical diagnostic tool. Ultrasound is also used for non-destructive testing and analysis in the technical arts. Medical ultrasonic transducer arrays typically operate at a frequency within the range of one MHz to ten MHz, although higher frequencies are certainly possible. A two-dimensional phased array of ultrasonic transducer elements is often designed to obtain image data in two or three dimensions, without requiring movement of the array transducer.
Medical ultrasonic transducer arrays conventionally are fabricated from a block of ceramic piezoelectric material within which individual elements are defined and isolated from each other by sawing at least partially through the block of piezoelectric material, making a number of cuts with a dicing saw.
More particularly, in the fabrication of a two-dimensional array, as a preliminary step a dicing saw is employed to make several isolation cuts (for example from three to eight isolation cuts) most of the way through the block of piezoelectric material to define isolated rows or subarrays. Subsequently, a second series of many (for example approximately 128) dicing saw cuts are made at right angles to the isolation cuts, typically all the way through the block of piezoelectric material, to define individual piezoelectric array elements within each row or subarray. Each resultant piezoelectric element is acoustically and electrically isolated from its neighbors.
The dicing saw cuts are made with specialized equipment, in particular an automatic dicing saw intended for precision cutting of electronic materials including silicon, glass, and ceramics with a diamond abrasive saw blade.
One approach to providing individual electrical signal connections to the small piezoelectric elements defined by dicing is disclosed in Smith et al. U.S. Pat. No. 5,091,893, issued Feb. 25, 1992 and assigned to the instant assignee. The entire disclosure of U.S. Pat. No. 5,091,893 is hereby expressly incorporated by reference.
Very briefly, U.S. Pat. No. 5,091,893 discloses an array ultrasonic transducer structure in which a high density interconnect (HDI) structure is employed for signal conductor connections to the individual piezoelectric transducer elements. The HDI structure includes a polyimide dielectric film layer, which may comprise Kapton polyimide at a thickness of about 0.001 to 0.003 inch (25 to 75 microns) and available from E.I. DuPont de Nemours & Company, Wilmington, Del. Parallel conductors comprising a patterned metallization layer are formed on the dielectric film layer and connected to the piezoelectric elements through via-holes in the dielectric film layer.
In the disclosure of U.S. Pat. No. 5,091,893, the HDI structure is fabricated directly on a body of piezoelectric material by a process including various laser drilling, metallization and patterning steps, to form an array ultrasonic transducer precursor. Insofar as the present invention is concerned, a similar array ultrasonic transducer precursor may be formed by separately forming a so-called flex circuit comprising a dielectric film layer with patterned conductors, and then laminating the flex circuit to the body of piezoelectric material.
In either event, what results is an array ultrasonic transducer precursor including a body of piezoelectric material with a dielectric substrate bonded to a surface thereof, and a plurality of physically parallel signal conductors arranged in a pattern supported on the dielectric substrate. Thus, in a typical transducer fabrication process, electrical connections between the signal conductors and what will ultimately become the individual piezoelectric elements are made prior to the dicing cuts which define the individual, isolated piezoelectric elements. The dicing saw not only cuts the piezoelectric material, forming a narrow kerf between adjacent elements, but also runs between the parallel signal conductor tracks, cutting away portions of the dielectric substrate. Dicing saw alignment is thus with reference to the conductor pattern supported by the flex circuit; the precise locations of the dicing saw cuts in the body of piezoelectric material are a result of the saw position with reference to the pattern of parallel signal conductors.
Extremely close tolerances are involved. As an example of transducer design employing relatively small elements, the saw kerfs are 40 microns wide, on a pitch of 300 microns. Between each pair of kerfs, two signal traces extend to the left and two to the right. Each trace is connected to one piezoelectric element by means of a via-hole through the flex circuit. A typical pad around the via-hole is 100 microns in diameter. A 60 micron wide gap exists between the via pads on the upper edge of one row of elements and the via pads on the lower edge of the next row of elements. If the dicing saw is to pass between via pads without damaging them, a tolerance of .+-.10 microns is required for the alignment of the saw with reference to the flex circuit.
However, as noted above, in a typical fabrication process, electrical connections are made early in the process, while the isolation is performed later, after the electrical connections have been encapsulated. The encapsulating materials are generally opaque, so it is difficult to precisely align the isolation cuts with the element pattern established by the electrical connections.
Prior to the present invention, optical alignment was relied upon, employing optical fiducial points or marks formed on a portion of the flex circuit which extends beyond the piezoelectric element and which is removed as a subsequent step in the fabrication process. An optical microscope is conventionally included as part of a dicing saw and is employed for alignment purposes. The optical fiducial points are precisely positioned with reference to the pattern of parallel signal conductors. Since the piezoelectric material is opaque, the optical fiducial points must be located far from the pattern of signal conductors, so that the optical fiducials are visible from the front of the transducer during the alignment and cutting process. After initial fixturing of an array ultrasonic transducer in the dicing saw, an optical alignment is performed by employing the microscope and the optical fiducials. Based on these measurements, rotational and translational offsets, as well as a translational pitch, are calculated for the isolation cuts relative to the fiducial points.
However, because the optical fiducial points are far from the pattern of signal conductors, distortions in the flex circuit during the transducer assembly and dicing processes lead to inaccuracy in the alignment of the saw blade relative to the signal conductors. As a result, the optical alignment method only allows positioning of the saw blade within .+-.30 micron accuracy.
For some transducer designs, particularly those employing relatively small elements, the yield when employing optical alignment is less than desirable. One reason is that the optical fiducials do not remain co-planar with the pattern of signal conductors, and may be distorted when viewed from the front or back of the transducer during the optical alignment process.